This application is related to my co-pending application Ser. No: 862,697 filed Apr. 3, 1992, now abandoned.
This invention pertains to heat recovery steam generators which contain both superheater and reheater tube units which are arranged thermally in parallel, and in which tubes for each unit are internested with each other within the steam generator.
Large high pressure utility boilers or steam generators often incorporate steam reheat cycles in which the steam which is partially spent by being expanded through a high pressure turbine, i.e. has substantial heat removed, is returned to the boiler for reheating and then is passed through a low pressure turbine. In such a manner, additional energy can be removed from each pound of steam generated, therby making the overall steam power cycle more efficient. For such heat recovery steam generators, the incorporation of a reheater unit provides some cycle efficiency advantages. The normal central station steam generator arrangement of placing the super heater in parallel with the reheater (parallel gas flow) results in minimum and thus ideal heat transfer surface area requirement. However, such parallel arrangement of superheater and reheater units causes some difficult structural problems. When reheat steam flow is not established during steam generator start-up, or when the temperature of the reheat steam does not match the parallel superheater during low power operation, then considerable temperature imbalance may occur throughout the generator and in the high pressure evaporator unit. The performance, fatigue and stress life of such a heat recovery steam generator design is suspect.
A common arrangement for overcoming these temperature imbalance problems is to alternate the superheater and reheater tube banks in the steam generator. However, considerable additional heat transfer surface is required for such a generator, because of the resulting poor temperature difference between the hot exhaust gas and the steam being heated is excessive, and can be as high as 60% additional surface. Although design refinements could reduce this additional heat transfer surface area requirement somewhat, it will always exceed the optimum area. Known heat recovery steam generators which utilize some useful tube arrangements are disclosed by U.S. Pat. No. 4,188,916 to Csathy; U.S. Pat. No. 4,664,067 to Haneda et al; U.S. Pat. No. 4,858,562 to Arakawa et al; and U.S. Pat. No. 4,944,252 to Motai et al; however further improvements are desired. The present invention provides a unique solution to this problem by providing an arrangement in which the superheat steam and reheat steam flow is alternated in each adjacent row of tubes.